There is a "huge possibility" that the missing Indonesia AirAsia Flight QZ8501, carrying 162 on board, has been in an accident, said Indonesian Vice-President Jusuf Kalla on Sunday.
Mr Kalla, speaking at a televised press conference on Sunday, also advised people to prepare for the worst.
"It has been 10 hours, there is a huge possibility there has been an accident.
"The government is concerned and expresses its deepest condolences to the families of victims," he said.
Malaysian Prime Minister Najib Razak also spoke of the possibility of the plane having crashed.
He said at a flood relief centre on Sunday that the plane disappearing from radar screens soon after departure "means the possibility of something untoward happening to the aircraft", reported The Star.
"That's all I know (about the incident) right now and we will assist in finding out what happened to the aircraft," he added.
Search and locate efforts for the missing aircraft resumed on Monday, with a second Republic of Singapore Air Force C-130 plane joining the search on Monday morning.